1. Field of the Invention
This invention relates to the art of electronic circuit packaging, and more specifically to the packaging of electronic integrated circuits into assemblies referred to as "memory modules" for large capacity or high density memory applications in high speed computer systems.
2. Description of the Prior Art
Computers over time have increased in functionality and decreased in size. Driving this has been the miniaturization of circuitry and the maximum utilization of space through clever and innovative packaging techniques. Additionally, computer systems have become faster and, as a result, the need for higher speed packaging solutions has become necessary. In computers there are two types of memory, read-only memory (ROM) and random access memory (RAM). RAM is used as a temporary work space for software applications to store data that is currently being worked on, such as documents, pictures and the like. Because of the evolution of more complex software applications, a requirement for larger and larger amounts of RAM has developed.
Computers are designed to accommodate the need for expansion of RAM by employing a packaging technique known as a memory module. Memory modules come in various configurations and types and are commonly known as SIMMs (Single Inline Memory Module), DIMMs (Dual Inline Memory Module), SODIMMs (Small Outline Dual Inline Memory Module), PCMCIA (Personal Computer Memory Card International Association), and MMC (Micro Memory Cards), to name a few. Memory modules are separate electronic assemblies comprised of a plurality of memory integrated circuits and support circuitry such as line drivers or buffers, clock drivers or interface logic that is mounted on a printed circuit board. The printed circuit board has a plurality of connecting pads along one edge so that these assemblies can be installed in the computer by plugging them into connectors or sockets. A number of sockets are placed on computer motherboards so that the computer user may easily add or upgrade their RAM.
The prior art has developed foldable electronic module assembly specifically to take advantage of the assembly on which to mount extra memory components and to minimize the space consumed by the assembly. An example of this technique is found in U.S. Pat. No. 5,224,023, the '023 patent, wherein a pair of quadrangular circuit boards are each laminarly split and mounted on both sides of a flexible insulating substrate having a network of conductive leads and connecting stations applied thereto. The half-sections of boards are adapted to have memory chips mounted thereon and sandwich the substrate therebetween. A pair of spaced-apart folds is made in the substrate so that it takes on an "S" shape that is squeezed together to save space. One of the outside half-sections of circuit boards has a comb of connecting terminals formed along one free edge that is plugged into a socket on the motherboard.
While this foldable electronic module assembly saves room inside the computer enclosure and allows the addition of RAM and other components to the already crowded interior of the computer system, it does not solve additional problems brought on by the demands of high performance, high speed computer systems. In the foldable module of the '023 patent, the memory components on each successive section are located greater and greater distances from the interconnect point with the motherboard. As the speed of the memory data bus is increased in computer systems, in order to increase the performance of the system, the successive increase in travel distance caused by the physical arrangement of the memory components of the '023 patent induces a propagation delay that mitigates or diminishes the signal integrity of the memory system. It is necessary in high performance memory systems to make the trace lengths as short as possible and as equal in length as possible. The invention described below solves the trace length problems found in memory modules of the type in the '023 patent or other types in the industry that are described as board-stacking memory modules.